Electroplated steel sheet

ABSTRACT

An electroplated steel plate product with a zinc-rich deposit on the steel surface, said deposit comprising an oxide of molybdenum and tungsten (including hydrated oxides), and/or the total amount in the oxide being about 0.05 to about 2 weight percent of the deposit. In addition, the deposit may further include at least one additional material selected from the group consisting of iron, nickel, cobalt, tin and lead, and compound of said metal, the total metal content of the said additional material being about 0.5 to about 15 weight percent of the deposit.

United States Patent [191 Ariga et al.

[ ELECTROPLATED STEEL SHEET [75] Inventors: Keiji Ariga; Satoshi Morishita;

Katsumi Kanda; Takashi Mizobe, all of Yamaguchi-Ken. Japan [73] Assignee: Toyo Kohan Co., Ltd., Tokyo, Japan [22] Filed: July 23, 1971 [21] Appl. No.: 165,751

[ 1 =,-,-122/.t1?61 1 3 ZLZ94L5M B. [51] Int. Cl. C23b 11/00, C23d 5/00 [58] Field of Search... 204/43 Z, 55 R, 56; 29/1965 [5 6] References Cited UNITED STATES PATENTS 2,233,500 3/1941 Westbrook 204/43 Z X 2,900,313 8/1959 Saubestre et al.. 204/43 Z 3,317,412 5/1967 Dahlmann 204/55 R FOREIGN PATENTS OR APPLICATIONS 1,043,618 9/1966 Great Britain 204/55 R [451 Feb. 12, 1974 Primary Examiner-F. C. Edmundson Attorney, Agent, or FirmGeorge B. Oujevolk [57] ABSTRACT An electroplated steel plate product with a zinc-rich deposit on the steel surface, said deposit comprising an oxide of molybdenum and tungsten (including hydrated oxides), and/or the total amount in the oxide being about 0.05 to about 2 weight percent of the deposit. In addition, the deposit may further include at least one additional material selected from the group consisting of iron, nickel, cobalt, tin and lead, and compound of said metal, the total metal content of the said additional material being about 0.5 to about 15 weight percent of the deposit.

2 Claims, No Drawings ELECTROPLATED STEEL SHEET BACKGROUND OF THE INVENTION The present invention relates to electroplated steel products and a method of producing an electroplated steel plate or sheet having excellent corrosion resistance and formability suitable for coating paint or synthetic resin enamel on the surface thereof.

OBJECT OF THE INVENTION Thus, it is an object of the present invention to provide a steel sheet or plate which, when coated with a paint, has excellent paint adhesion properties, as well as excellent corrosion resistance and formability when paints made of resins such as melamine, acrylic, epoxy, vinyl or others are applied onto the steel sheet.

BRIEF DISCUSSION OF THE PRIOR ART Zinc is electrochemically so active that the phenomenon of white rust or self dissolution of zinc coating easily occurs, especially in corrosive circumstances, such as wet, salty or acidic atmospheres. Though its corrosion resistance can be improved by applying some organic coatings, in many cases its paint adhesion property is not satisfactory.

Therefore, many attempts have been made to improve the above-mentioned defects of zinc coated steel surfaces. One of them is chemical or electrolytical posttreatment, and another is the galvannealing process.

Conventional, well-known methods of posttreatment generally comprise:

l. Dipping of zinc coated steel in a chromic acid base or phosphoric acid base solution.

2. Coating a zinc coated surface with a solution of chromic acid base or phosphoric acid base.

3. Treating the zinc or zinc alloy surface in a solution which has a high pH value on the alkaline side (more than 11) and contains at least one metal ion selected from the group consisting of silver, magnesium, cadmium, molybdenum, aluminum, titanium, chromium and others, and further containing enough complexing agent to dissolve the metal ions (Japanese Patent No. 531221).

These treatments will prevent white rust and enhance paint adhesion, but the treatments are effective for only a short period of time. The reason is that the paint adhesion quickly deteriorates because the corrosion of the zinc coating is accelerated by mechanical damage or chemical self-deterioration of the treated film. So it is conventional to overcoat the surface with organic finishes.

On the other hand, the galvannealed steel sheet, (the surface layer of which consists of a zinc-iron alloy) has better paint adhesion and corrosion resistance than a conventional electrogalvanized steel sheet, but a zinciron alloy layer has rather poor formability, and therefore, after forming, the paint adhesion deteriorates because of the fractures of the zinc-iron alloy layer.

The present invention is based upon the technological discovery that the passive properties of a zinc-rich deposit with at least one oxide of molybdenum and/or tungsten is effective to restrain the dissolution of zinc under corrosive conditions so that excellent corrosion resistance and paint adhesion can be obtained.

SUMMARY OF THE INVENTION Generally speaking, the present invention contemplates providing an electroplated steel product with a zinc-rich deposit on the steel surface, the deposit comprising (1) an oxide (and as used herein, this term includes hydrated oxides) of molybdenum and tungsten, and/or (2) in addition, the deposit may also include at least one of the metals or metal compounds selected from the group consisting of iron, nickel, cobalt, tin and lead, so as to give to the finished product excellent corrosion resistance and formability. As the base metal employed in the present invention, the plate, cut sheet,

. or strip of steel or iron is most convenient, and the term base metal, as used herein, refers to such a plate, cut sheet, or strip of steel or iron, which hereinafter for convenience, is referred to as a sheet.

The electroplated steel sheet contemplated herein is produced by cathodically electroplating thesteel sheet in an aqueous solution which comprises mainly water soluble, or bath soluble zinc compounds, and at least one compound of molybdenum or tungsten. Or, another method of producing the electroplated steel sheet is by cathodically electroplating the steel sheet in an aqueous solution which comprises mainly a water soluble, or bath soluble zinc compound, and at least one compound of molybdenum or tungsten, and further includes at least one water soluble or bath soluble metal compound of iron, nickel, cobalt, tin or lead.

DETAILED DESCRIPTION The composition of the deposit of the present invention is classified into two main groups, namely, (1) the deposit which comprises at least one of the oxides of molybdenum and tungsten, the total metal amount in said oxides being about 0.05 to about 2 weight percent of the deposit, and the balance zinc, and (2) the deposit which comprises at least one of the oxides of molybdenum and tungsten, the total metal amount in said oxides being about 0.05 to about 2 weight percent of the deposit, and further includes at least one metal or metal compound of the group consisting of iron, nickel, cobalt, tin and lead, the total metal amount of the additional metal or metal compound being about 0.5 to about 15 weight percent of the deposit, and the balance zinc.

The electroplated steel sheet produced according to the present invention can be surface coated with conventional coating materials, such as paint, varnish, lac quer and other organic finishes, among which are acryl based paint, melamine based paint, epoxy based paint, vinyl based paint, vinyl chloride based paint, oil based paint and other paints.

As to the methods of applying these coating materials to the electroplated steel sheet of the present invention, there can be utilized roller coating, spray coating, knife coating, laminating, injection coating, electrostatic coating, and others.

The electroplated steel sheet according to the present invention is produced by the following method.

The steel sheet is subjected to a conventional pretreatment such as pickling with acids after degreasing with alkalis, or pickling with acids only, or degreasing with alkalis only, and rinsing with water. Then, the steel sheet is put into an electrolytic bath and cathodically plated. The composition of the bath is mainly water soluble or bath soluble zinc compounds, (for example,

zinc sulfate, zinc chloride, zinc acetate, zinc fluoborate) and at least one water soluble or bath soluble compound of molybdenum and tungsten. (For exam ple, ammonium molybdate, sodium molybdate, potassium molybdate, ammonium tungstate, sodium tungstate, potassium tungstate).

Also, another bath composition for the present invention is mainly water soluble or bath soluble zinc compounds, at least one water soluble or bath soluble 'compound of molybdenum and tungsten, and further,

at least one water soluble or bath soluble compound of iron, nickeLcobalt, tinor lead, (for example, iron sulfate, nickel sulfate, cobalt sulfate, stannous sulfate, iron nitrate, nickel nitrate, cobalt nitrate, lead nitrate, iron chloride, nickel chloride, cobalt chloride, tin chloride, cobalt acetate, lead acetate).

However, as the composition of the deposit changes owing to current density, bath temperature, bath concentration, degree of agitation and other factors, the electrolysis operation should be controlled so that the composition of deposit is within the range of the composition of deposit of the present invention. The appearance of the treated surface with regard to the deposit formed changes with the kind or amount of the additional element. The color of the deposit changes depending upon the conditions of electrolysis, for example, blue, gray, brown or black. The deeper the degree of tint, the better the corrosion resistance, the formability and paint adhesion. However, if the composition of the deposit is not within the range of the composition of the present invention, the formability becomes poor, as hereinafter described.

For the purpose of giving those skilled in the art a better understanding of the invention, the following illustrative examples are given.

EXAMPLE 1 A 0.45 mm thick low carbon steel sheet was cathodically cleaned for 15 seconds at a current density of 3 A/sq.dm at 70C in a sodium hydroxide (70 g/liter) solution, and rinsed with water, then pickled for 5 seconds at room temperature in a sulfuric acid (70 g/liter) solution, and again rinsed with water. Then the steel sheet was put in an electrolytic bath and direct current was applied, wherein the steel sheet acted as the cathode and zinc acted as the anode. The composition of the bath was: zinc sulfate 250 g/liter, aluminum sulfate 20 g/liter, ammonium molybdate 4 g/liter and sodium sulfate 30 g/liter. The temperature was 45C, current density was 20 A/sq.dm and treating time was 16 seconds. The amount of the deposit formed was g/sq.m. The color of the deposit formed was blue-gray. As a result of the electrondiffraction analysis, it was established that the deposit formed was composed of Zn, Zn (OH) ZnO, M00 M0 0 and minor amounts of constituents. According to the quantitative analysis, the total molybdenum metal amount in the molybdenum oxides was'0.5 weight percent of the deposit. The properties of the sample are indicated as No. 5 in the Table given hereinafter.

EXAMPLE 2 The same low carbon steel sheet as described in Example l was subjected to the same pre-treatment, as described in Example 1. After that, the steel sheet was cathodically treated for seconds at 45C at a current density of A/sq.dm in an electrolyte containing 250 g/liter zinc sulfate, 0.5 g/liter ammonium molybdate and 30 g/liter cobalt sulfate. The amount of the deposit formed was 10 g/sq.m. From the result of the electrondiffraction analysis, it was found that the deposit formed was composed of Zn, Zn (OH) ZnO, M00 M0 0 Co(OH) C00, and other minor constituents, and the total molybdenum metal amount in the molybdenum oxides and total cobalt metal amount was 0.5 and 3.0 weight percent of the deposit, respectively.

The salt spray test provided by 118 (Japanese lndustrial Standard) Z 2371 was applied (first) to the electroplated steel sheet produced according to the present invention; (second) to a conventional zinc-cobalt alloy plated steel sheet (coating weight 10 g/sq.m) containing 2 weight percent of cobalt; and, (third) to a conventional electrogalvanized steel sheet (coating weight 10 g/sq.m). The (first) electroplated steel sheet according to the present invention showed no red, rusty spots after 12 hours; in the case of the (third) conventional electrogalvanized steel sheet, to percent of the total surface was covered with red, rusty spots. The (second) conventional zinc-cobalt alloy plated steel sheet showed rusty spots which covered 50 percent of the total surface. From the results of successive tests, the electroplated steel sheet according to the present invention was proved to withstand about 4 times longer in corrosion resistance tests in comparison with conventional electrogalvanized steel sheets.

The conventional electrogalvanized steel sheet was then treated in a phosphate based solution and rinsed with water, then treated in the chromate based solution. The conventional electrogalvanized steel sheet, which has 20 mg/sq.dm of phosphate film on the surface by the conventional phosphating treatment and 0.2 mg/sq.dm of elemental chromium in chromate film overlaid by the conventional chromating treatment, revealed as many blisters as those of the zinc-cobalt alloy plated steel sheet after 500 hours of paint adhesion test described in (c) in Note: 3 of the Table, whereas the electroplated steel sheet according to the present invention showed no evidence of blisters.

These same samples were coated with 250 mg/sq.dm of a melamine based paint and cross hatched on the paint film baked, then immersed in running water for half a year, after that period, a piece of adhesive tape was applied firmly on the surface thereof and then pulled off quickly. whereupon the conventional electrogalvanized steel sheet showed many blisters and adhesion loss of paint film was 20 percent. In case of the zinc-cobalt alloy plated steel sheet there were many blisters and adhesion loss of paint film was 15 percent. On the other hand, the electroplated steel sheet according to the present invention could withstand the running water exposure test for more than one and a half year, without any signs of blisters and adhesion loss of paint film did not occur.

Furthermore, it was found that the electroplated steel sheet according to the present invention, which is coated with 70 mg/sq.dm of acryl based adhesive agent and then with 0.2 mm thick polyvinyl chloride thereon, had much higher resistance to the degradation of film adhesion in the said running water exposure test than the conventional electrogalvanized steel sheet. The properties of this sample are indicated as No. l l in the Table.

EXAMPLE 3 Low carbon steel sheet, as described in Example, 1

was subjected to the same pre-treatment as described in Example 1. Then, the steel sheet was cathodically ample 1 was subjected to the same pre-treatment as described in Example I. Then the steel was cathodically treated for 16 seconds at 45C at a current density of 20 A/sq.dm in the electrolyte containing 60 g/liter of treated for 16 seconds at 45C at a current density of 5 zinc f 150 gmter of Zinc q glut of 20 A/sq.dm in an electrolyte containing 250 g/liter of ifz fig z 60 Elmer of i: zinc sulfate, 0.5 g/liter of ammonium molybdate and e co or 0 e eposit 0mm a 05 glut of Sodium tungstaw The amount of the of the electron-diffraction analysis, it was found that posit formed was g/sq.m. The color of the deposit gs s g g g g -"gg ig 9 3" 95 3 formed was blue-gray. As a result of electon-diffraction l0 and a, g yz g filgg iz t 2; g: 2 ?gg ggg a gg j gg molybdenum oxides and total metal amount of lead was 5v 0 and other minor constituents a rid the trital m and 5 w-eight pefcem of the dpo-sil' respectively yg gfi and tungsten metal amount in the Said m0 The properties of this sample are indicated as No. 22

. 15 in the Table.

gg i g ggsz xf s fsi i g ii g 25 22 Regarding the electroplated steel sheet according to ti 5 1 d P N Z1 th L the present invention, the relationship between the var- 1 samp e are m 6 as m e a ious compositions of the deposit and their properties EXAMPLE 4 are indicated in the Table.

The low carbon steel sheet as described in Example In the case ofgroup Ofthe combmatlonfomppsb l was subjected to the same pre-treatment as described i zg gigig zi ggig f w grj 5:3 ?:2232: in Example 1. After that, the steel sheet was cathodil bdenum Oxides i as low as 0 O4 ht ercent of cally treated for 16 seconds at 45C at a current density j de 08 (listed as No 3 in the'Tablwgthepmol of 20 A/sq.dm in the electrolyte containing 250 g/liter h d ff d y of zinc sulfate, 0.5 g/liter of ammonium molybdate and 25 g 1 i the pzfpemeshwere poor 50 g/liter of nickel sulfate. The amount of the deposit i a accorHmg to t 6 s formedwas l0 g/sqm. The color of the deposit formed nona g g processf' rg t e was gray. As a result of the electron-diffraction analy- 22 33 .3 l jz z j fi 3 gj z fi g s g; thazt ii g fa g iff coglpoied ing to the present invention resulted in excellent corroz s ki 2 tztal 2 i' i s; sion resistance and good resistance to the degradation of paint adhesion. However, in case of more than 2 amontmthe dmlbd m th t a t t of the deposit respectively. The properties of this sam- 35 2 3 25 as O m e a e) revea 6 poor orm ple are mdlcated as 21 m the Table In the case of group (2) of the combination of com- E AM E 5 position of deposit according to the present invention,

th t t l 1 bd l t' th The same low carbon steel sheet as described in Ex- $232 g g i if gf j 2 23:; S

2F 2; n g ifi iz fflfigggig:gizg gz ggfzgx ii' 40 posit, and further, the total iron metal amounted to 15 cally treated for l 6 seconds at C at a current density K a g g iz g i g f fi s 'g' iste as o. 0. ,an o. mte a e,ac-

g; g /l z 2?:giiggfifiiigfiyiigg 2 :5 cording to the present invention, an excellent effect for improving the properties of deposits was found.

: g g f g g i g g fgff fig 5 Namely, the existence of the molybdenum oxides and 9" a S F re u h e S metal or compounds of iron resulted in excellent resisg if i g gg 5 11 555 p 2 5 ztnf) mg" tance against the degradation of paint adhesion in comparlson with Sample No. 4 or No. 2 1n the Table acxgfgii ii z g g g g gaz ig ig zg sgfiygf g rzl cording to the conventional zinc iron alloy plating prooxides and the total metal amount of iron was 0.7 and 50 2 2: 23: 1:32 s zfis zgzfi t i zi ig g g 3 :f Fj i the ifg g gs gg E 3 weight percent of the deposit, if the total iron metal er 0 18 p e are l amount exceeded 15 weight percent of the deposits, as E 6 shown in No. 18 in the Table, the effect for the resis- EXA L 55 tance against the degradation of paint adhesion was sat- The same low carbon steel sheet as described in Exurated andconversely the formability became worse.

TABLE Film Composition Properties (b) Formability (c) Paint Adhesion (a) Corrosion No Metal Component Weight Percent Resistance Performance l Zn 100 C A 2 Zn-Fe -10 C C C43 3 Zn-Mo 99.96-0.04 C A C 4 Zn-Mo 99.95-005 B A B 5 Zn-Mo 9950-050 B A A 6 Zn'Mo 97 .50-2.50 A a C .A

Table;- C o rninued Film Composition Properties No Metal Component Weight Percent (a) Corrosion (b) Formability (c) Paint Adhesion Resistance Performance 7 Zn-Mo-W 99.95- 02-003 B A B-A 8 Zn-Mo-W 99.70-0 l0-0.20 B-A A B-A 9 Zn-Mo-W 99.30-0 50-010 8 A B-A l0 Zn-Mo-Co 99.45-0 -050 B-A A B-A ll Zn-Mo-Co 96.50-0.S0-3.00 B A A 12 Zn-Mo-Co 84.95-0.05-l5.00 A A A 13 Zn-Mo-Co 82.95-0.05-l7.00 A C A 14 Zn-Mo-W-Co 99.30-0.l0-O.l0-0.50 A A A 15 Zn-Mo-Fe 99.45-0.05-0.50 B A B-A l6 Zn-Mo-Fe 96.30-0.70-3.00 B A A l7 Zn-Mo-Fe 84.95-0.05-l5.00 B A A 18 v Zn-Mo-Fe 80.50-0.50-l9.00 B C A 19 Zn-MoNi 99.70-010-020 B-A A A 20 Zn-Mo-Ni 99.90-0.05-0.05 B-A A B-A 2l Zn-Mo-Ni 97.70-0.30-2.0 B A A 22 Zn-Mo-Pb 94.80-0.20-5.0 B A B-A Note: I The deposit weight of Samples No.5, No. 9, No. ll. No. 16, No. 21, and No. 22 was It] g/sq.m. The

deposit weight of the other Samples was g/sq.m.

Note: 2 Samples No. l and No. 2 in the Table were treated in the phosphate based solution. The weight of the Phosphate film was 1 g/sq.m. Note: 3 The method for testing properties in the Table:

a. Corrosion Resistance The salt spray test provided by 11S (Japanese Industrial Standard) 2 237l was practiced for those samples, and the red rust spots developed after I00 hours was observed. Test results are classified into 3 classes: A,-B, and C. A means that no area of rusty spots was found.

B means that the area of rusty spots was 20 to percent.

C means that the area of rusty spots was 80 to I00 percent.

b. Formability Samples were coated with 200 mg/sqm of alkyl based paint and deeply drawn to the cup (drawing ratio: 2.0) and a piece of adhesive tape was applied firmly on the deeply drawn part of the cup and then pulled off quickly. Test results are classified into 3 classes: A, B, and C.

A means that no adhesion lossof the coated paint was found. 8 means that adhesion loss of the coated paint was 20 to 40 percent. C means that adhesion loss of the coated paint was 80 to 100 percent.

c. The Resistance Against the Degradation of Paint Adhesion Samples were coated with 250 mglsq.rn of melamine based paint, cross hatched with a razor on the paint film and subjected to the salt spray test A, B, and C.

A means that no blisters were found on the cross hatched part.

B means that some blisters were found on the cross hatched part. .C means that many blisters were found on the cross hatched part.

One of the reasons why the electroplated steel sheet, according to the present invention, has excellent resistance against degradation of paint adhesion, has good formability, and good corrosion resistance, is because of the effect of metal oxides in the deposit formed. The behavior of the metal oxides and metal compounds which produce the excellent properties hereinbefore described is believed to be that when some corrosive substances permeate through the paint layer perpendicularly to the electroplated steel sheet, the oxides and compounds of such metals as molybdenum, tungsten, iron, nickel, cobalt, tin, and lead are effective to prevent the propagation of zinc dissolution along the horizontal directions and thus prevent the direct reaction of the steel base with corrosive substances.

We claim:

1. An electroplated steel product having a zinc-rich deposit onthe steel surface, said product is formed in for 500 hours according to HS 2 2371. Then, blisters on the cross hatched part were observed. The results are classified into 3 classes:

an acidic aqueous electrolytic bath, and said deposit comprising at least one of the oxides, of molybdenum and tungsten, the total amount of metal in said oxide being from 0.05 to 2 weight percent of the deposit.

2. An electroplated stccl product with a zinc-rich deposit on the steel surface, said deposit comprising at least one oxide material selected from the group consisting of the oxides of molybdenum and tungsten, the total amount of molybdenum and tungsten being about 0.05 to about 2 weight percent of the deposit, and further, including at least one additional material selected from the group consisting of iron, nickel, cobalt, tin and lead, and oxides and hydroxides of said metals, the total metal content of the said additional material being about 0.5 to about 15 weight percent of the deposit said product having been formed by electro-deposition in an acidic aqueous electrolytic bath. 

2. An electroplated steel product with a zinc-rich deposit on the steel surface, said deposit comprising at least one oxide material selected from the group consisting of the oxides of molybdenum and tungsten, the total amount of molybdenum and tungsten being about 0.05 to about 2 weight percent of the deposit, and further, including at least one additional material selected from the group consisting of iron, nickel, cobalt, tin and lead, and oxides and hydroxides of said metals, the total metal content of the said additional material being about 0.5 to about 15 weight percent of the deposit said product having been formed by electro-deposition in an acidic aqueous electrolytic bath. 